There is a continuing need to improve procedures for docking one space vehicle to another space vehicle. Once these space vehicles are in close proximity to each other, some current systems rely on a pilot in one of the vehicles who makes use of radar and/or vision systems while steering one of the vehicles relative to the other vehicle. Unfortunately, there are many factors that cause this procedure to be done in less than ideal conditions. First of all, the pilot is typically not able to directly view the portions of the vehicles that are being docked together, such as through a window. Instead, the pilot is typically viewing a radar or video display. In the case of a video display, the operator is viewing a two-dimensional image supplied by a camera while performing a three-dimensional operation. Further, the image quality may suffer due to lighting constraints. For example, the ambient sunlight may be blocked by the earth or by either vehicle. Alternatively, the camera may be looking toward the sun during the docking operation. Also, it is possible for the optics to become fogged due to moisture or contaminated due to chemical deposition, thus washing out the image.
Radar-assisted docking also has drawbacks. First of all, the pilot may not be as comfortable maneuvering the space vehicle while viewing only radar imaging as opposed to video imaging or direct viewing. Further, the measurement by the radar of range to the other vehicle, range rate, and direction may not be sufficiently precise. Also, there is typically not much in the way of attitude or angular orientation information available from radar imaging systems.
As an alternative to having the docking operation performed by a pilot in one of the vehicles, the docking operation could be performed by ground operators. Unfortunately, the transmission delays inherent in transmitting video, range, range rate, and direction information to the ground and then transmitting command and control information from the ground operator back to one of the space vehicles make such a procedure difficult. Any type of control loop would be negatively affected by such delays. Further, there are few opportunities for direct communication between the space vehicles and the ground operators. When direct communication from the space vehicle to the ground operators is not possible, then additional transmission delays are incurred due to satellite relays. Of course, these additional transmission delays exacerbate the problem.
There is also at least one automated system in use for docking space vehicles. This system (known as the KURS system) is employed by Russian-built Soyuz and Progress space vehicles in docking with the International Space System (ISS). The KURS system includes an RF beacon signal sent by the ISS and a receiving antenna on the Soyuz vehicle. The Soyuz antenna is mechanically spun about an axis and the received signal from the beacon will be constant if the beacon signal originates on the docking axis. The received signal will be amplitude modulated at approximately 4% for every degree the beacon signal is off-axis. As can be appreciated, the Soyuz vehicle can thus be controlled to keep the beacon (and thus the ISS) on axis via a control loop that controls the Soyuz vehicle to move in a direction to minimize the amount of amplitude modulation in the signal. An additional complement of similar equipment provides similar measurements from the ISS perspective. Additionally, there is a set of antennas and electronics that separately perform round trip range and range rate measurements.
In addition to radar and vision systems, various laser-ranging systems have been proposed and developed. Unfortunately, such systems suffer in their maximum working range. Most, if not all, such systems may only be useable within a range of less than 1 to 2 kilometers while RF-based systems may be usable in the 40 to 60 kilometer range or more. Since it is desirable to minimize the weight of equipment employed on space vehicles, it may be counter-productive to have an optical system with such a limited working range that a secondary system is also required for greater distances.
The problems of limited range and other problems associated with current systems are exacerbated by the fact that at any given moment the exact location of an orbiting vehicle such as the ISS may have a great deal of uncertainty associated therewith. This uncertainty can result from the fact that the ISS is moving at 17,000 MPH or 7.6 meters per millisecond, so it can be appreciated that position estimates are more difficult with this change in position per unit time. Further, the uncertainty is affected by the irregular orbit of the ISS due to gravity variations. Since the mass of the earth is not uniformly distributed, the gravitational force experienced by an orbiting space vehicle is not constant. This causes the radius of the ISS' orbit to vary, as do solar events and other environmental factors. This variation in the orbital radius is particularly a factor for low earth objects such as the ISS.
A Japanese system known as ETS-7 was tested in 1998 with two satellites in space. The system included a combination of a GPS receiver, laser radar (with a pulsed laser), and a proximity sensor. The GPS receiver assisted with position locating from a range of 10 km in to 600 m, the laser radar was used from 660 m in to 2 m, and the proximity sensor was used inside of 2 m. The proximity sensor included approximately 100 LEDs and a CCD camera.
All of the above challenges must be considered in light of the potentially disastrous consequences that could occur if a mishap takes place during a docking procedure. As can be appreciated, the consequences can vary from injury to the occupants of the space vehicles to damaging very expensive equipment for which great expense has been incurred to place them in orbit.
It is against this background and with a desire to improve on the prior art that the present invention has been developed.